The present invention relates to methods for driving electro-optic displays, especially bistable electro-optic displays, and to apparatus for use in such methods. More specifically, this invention relates to driving methods which are intended to enable a plurality of drive schemes to be used simultaneously to update an electro-optic display. This invention is especially, but not exclusively, intended for use with particle-based electrophoretic displays in which one or more types of electrically charged particles are present in a fluid and are moved through the fluid under the influence of an electric field to change the appearance of the display.
The term “electro-optic”, as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The term “gray state” is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate “gray state” would actually be pale blue. Indeed, as already mentioned the transition between the two extreme states may not be a color change at all.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
The term “impulse” is used herein in its conventional meaning of the integral of voltage with respect to time. However, some bistable electro-optic media act as charge transducers, and with such media an alternative definition of impulse, namely the integral of current over time (which is equal to the total charge applied) may be used. The appropriate definition of impulse should be used, depending on whether the medium acts as a voltage-time impulse transducer or a charge impulse transducer.
Much of the discussion below will focus on methods for driving one or more pixels of an electro-optic display through a transition from an initial gray level to a final gray level (which may or may not be different from the initial gray level). The term “waveform” will be used to denote the entire voltage against time curve used to effect the transition from one specific initial gray level to a specific final gray level. Typically such a waveform will comprise a plurality of waveform elements; where these elements are essentially rectangular (i.e., where a given element comprises application of a constant voltage for a period of time); the elements may be called “pulses” or “drive pulses”. The term “drive scheme” denotes a set of waveforms sufficient to effect all possible transitions between gray levels for a specific display.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. Nos. 6,301,038; 6,870.657; and 6,950,220. This type of medium is also typically bistable.
Another type of electro-optic display is an electro-wetting display developed by Philips and described in Hayes, R. A., et al., “Video-Speed Electronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003). It is shown in U.S. Pat. No. 7,420,549 that such electro-wetting displays can be made bistable.
Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y, et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patent Publication No. 2005/0001810; European Patent Applications 1,462,847; 1,482,354; 1,484,635; 1,500,971; 1,501,194; 1,536,271; 1,542,067; 1,577,702; 1,577,703; and 1,598,694; and International Applications WO 2004/090626; WO 2004/079442; and WO 2004/001498. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in the these patents and applications include:                (a) Electrophoretic particles, fluids and fluid additives; see for example U.S. Pat. No. 7,002,728 and U.S. Patent Application Publication No. 2007/0146310;        (b) Capsules, binders and encapsulation processes; see for example U.S. Pat. Nos. 6,922,276 and 7,411,719;        (c) Films and sub-assemblies containing electro-optic materials; see for example U.S. Pat. No. 6,982,178 and U.S. Patent Application Publication No. 2007/0109219;        (d) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see for example U.S. Pat. No. 7,116,318 and U.S. Patent Application Publication No. 2007/0035808;        (e) Color formation and color adjustment; see for example U.S. Pat. No. 7,075,502 and U.S. Patent Application Publication No. 2007/0109219;        (f) Methods for driving displays; see for example U.S. Pat. Nos. 5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,116,466; 7,119,772; 7,193,625; 7,202,847; 7,259,744; 7,304,787; and 7,312,794; and U.S. Patent Applications Publication Nos. 2003/0102858; 2005/0024353; 2005/0062714; 2005/0122284; 2005/0152018; 2005/0179642; 2005/0212747; 2005/0253777; 2005/0280626; 2006/0038772; 2006/0139308; 2006/0139310; 2006/0139311; 2006/0181492; 2006/0181504; 2006/0197738; 2006/0232531; 2006/0262060; 2007/0013683; 2007/0091418; 2007/0103427; 2007/0200874; 2008/0024429; 2008/0024482; 2008/0048969; 2008/0054879; 2008/0117495; 2008/0129667; 2008/0136774; and 2008/0150888, and any other MEDEOD applications and patents mentioned above;        (g) Applications of displays; see for example U.S. Pat. No. 7,312,784 and U.S. Patent Application Publication No. 2006/0279527; and        (h) Non-electrophoretic displays, as described in U.S. Pat. Nos. 6,241,921; 6,950,220; and 7,420,549.        
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and published US Application No. 2002/0075556, both assigned to Sipix Imaging, Inc.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.
The bistable or multi-stable behavior of particle-based electrophoretic displays, and other electro-optic displays displaying similar behavior (such displays may hereinafter for convenience be referred to as “impulse driven displays”), is in marked contrast to that of conventional liquid crystal (“LC”) displays. Twisted nematic liquid crystals are not bi- or multi-stable but act as voltage transducers, so that applying a given electric field to a pixel of such a display produces a specific gray level at the pixel, regardless of the gray level previously present at the pixel. Furthermore, LC displays are only driven in one direction (from non-transmissive or “dark” to transmissive or “light”), the reverse transition from a lighter state to a darker one being effected by reducing or eliminating the electric field. Finally, the gray level of a pixel of an LC display is not sensitive to the polarity of the electric field, only to its magnitude, and indeed for technical reasons commercial LC displays usually reverse the polarity of the driving field at frequent intervals. In contrast, bistable electro-optic displays act, to a first approximation, as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied and the time for which this field is applied, but also upon the state of the pixel prior to the application of the electric field.
Whether or not the electro-optic medium used is bistable, to obtain a high-resolution display, individual pixels of a display must be addressable without interference from adjacent pixels. One way to achieve this objective is to provide an array of non-linear elements, such as transistors or diodes, with at least one non-linear element associated with each pixel, to produce an “active matrix” display. An addressing or pixel electrode, which addresses one pixel, is connected to an appropriate voltage source through the associated non-linear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this arrangement will be assumed in the following description, although it is essentially arbitrary and the pixel electrode could be connected to the source of the transistor. Conventionally, in high resolution arrays, the pixels are arranged in a two-dimensional array of rows and columns, such that any specific pixel is uniquely defined by the intersection of one specified row and one specified column. The sources of all the transistors in each column are connected to a single column electrode, while the gates of all the transistors in each row are connected to a single row electrode; again the assignment of sources to rows and gates to columns is conventional but essentially arbitrary, and could be reversed if desired. The row electrodes are connected to a row driver, which essentially ensures that at any given moment only one row is selected, i.e., that there is applied to the selected row electrode a voltage such as to ensure that all the transistors in the selected row are conductive, while there is applied to all other rows a voltage such as to ensure that all the transistors in these non-selected rows remain non-conductive. The column electrodes are connected to column drivers, which place upon the various column electrodes voltages selected to drive the pixels in the selected row to their desired optical states. (The aforementioned voltages are relative to a common front electrode which is conventionally provided on the opposed side of the electro-optic medium from the non-linear array and extends across the whole display.) After a pre-selected interval known as the “line address time” the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed so that the next line of the display is written. This process is repeated so that the entire display is written in a row-by-row manner.
It might at first appear that the ideal method for addressing such an impulse-driven electro-optic display would be so-called “general grayscale image flow” in which a controller arranges each writing of an image so that each pixel transitions directly from its initial gray level to its final gray level. However, inevitably there is some error in writing images on an impulse-driven display. Some such errors encountered in practice include:                (a) Prior State Dependence; With at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends not only on the current and desired optical state, but also on the previous optical states of the pixel.        (b) Dwell Time Dependence; With at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends on the time that the pixel has spent in its various optical states. The precise nature of this dependence is not well understood, but in general, more impulse is required the longer the pixel has been in its current optical state.        (c) Temperature Dependence; The impulse required to switch a pixel to a new optical state depends heavily on temperature.        (d) Humidity Dependence; The impulse required to switch a pixel to a new optical state depends, with at least some types of electro-optic media, on the ambient humidity.        (e) Mechanical Uniformity; The impulse required to switch a pixel to a new optical state may be affected by mechanical variations in the display, for example variations in the thickness of an electro-optic medium or an associated lamination adhesive. Other types of mechanical non-uniformity may arise from inevitable variations between different manufacturing batches of medium, manufacturing tolerances and materials variations.        (f) Voltage Errors; The actual impulse applied to a pixel will inevitably differ slightly from that theoretically applied because of unavoidable slight errors in the voltages delivered by drivers.        
General grayscale image flow suffers from an “accumulation of errors” phenomenon. For example, imagine that temperature dependence results in a 0.2 L* (where L* has the usual CIE definition:L*=116(R/R0)1/3−16,where R is the reflectance and R0 is a standard reflectance value) error in the positive direction on each transition. After fifty transitions, this error will accumulate to 10 L*. Perhaps more realistically, suppose that the average error on each transition, expressed in terms of the difference between the theoretical and the actual reflectance of the display is ±0.2 L*. After 100 successive transitions, the pixels will display an average deviation from their expected state of 2 L*; such deviations are apparent to the average observer on certain types of images.
This accumulation of errors phenomenon applies not only to errors due to temperature, but also to errors of all the types listed above. As described in the aforementioned U.S. Pat. No. 7,012,600, compensating for such errors is possible, but only to a limited degree of precision. For example, temperature errors can be compensated by using a temperature sensor and a lookup table, but the temperature sensor has a limited resolution and may read a temperature slightly different from that of the electro-optic medium. Similarly, prior state dependence can be compensated by storing the prior states and using a multi-dimensional transition matrix, but controller memory limits the number of states that can be recorded and the size of the transition matrix that can be stored, placing a limit on the precision of this type of compensation.
Thus, general grayscale image flow requires very precise control of applied impulse to give good results, and empirically it has been found that, in the present state of the technology of electro-optic displays, general grayscale image flow is infeasible in a commercial display.
Under some circumstances, it may be desirable for a single display to make use of multiple drive schemes. For example, a display capable of more than two gray levels may make use of a gray scale drive scheme (“GSDS”) which can effect transitions between all possible gray levels, and a monochrome drive scheme (“MDS”) which effects transitions only between two gray levels, the MDS providing quicker rewriting of the display that the GSDS. The MDS is used when all the pixels which are being changed during a rewriting of the display are effecting transitions only between the two gray levels used by the MDS. For example, the aforementioned U.S. Pat. No. 7,119,772 describes a display in the form of an electronic book or similar device capable of displaying gray scale images and also capable of displaying a monochrome dialogue box which permits a user to enter text relating to the displayed images. When the user is entering text, a rapid MDS is used for quick updating of the dialogue box, thus providing the user with rapid confirmation of the text being entered. On the other hand, when the entire gray scale image shown on the display is being changed, a slower GSDS is used.
More specifically, present electrophoretic displays have an update time of approximately 1 second in grayscale mode, and 500 milliseconds in monochrome mode. In addition, many current display controllers can only make use of one updating scheme at any given time. As a result, the display is not responsive enough to react to rapid user input, such as keyboard input or scrolling of a select bar. This limits the applicability of the display for interactive applications. Accordingly, it is desirable to provide drive means and a corresponding driving method which provides a combination of drive schemes that allow a portion of the display to be updated with a rapid drive scheme, while the remainder of the display continues to be updated with a standard grayscale drive scheme.
One aspect of the present invention relates to data structures, methods and apparatus for driving electro-optic displays which permit rapid response to user input. The aforementioned MEDEOD applications describe several methods and controllers for driving electro-optic displays. Most of these methods and controllers make use of a memory having two image buffers, the first of which stores a first or initial image (present on the display at the beginning of a transition or rewriting of the display) and the second of which stores a final image, which it desired to place upon the display after the rewrite. The controller compares the initial and final images and, if they differ, applies to the various pixels of the display driving voltages which cause the pixels to undergo changes in optical state such that at the end of the rewrite (alternatively called an update) the final image is formed on the display.
However, in most of the aforementioned methods and controllers, the updating operation is “atomic” in the sense that once an update is started, the memory cannot accept any new image data until the update is complete. This causes difficulties when it is desired to use the display for applications that accept user input, for example via a keyboard or similar data input device, since the controller is not responsive to user input while an update is being effected. For electrophoretic media, in which the transition between the two extreme optical states may take several hundred milliseconds, this unresponsive period may vary from about 800 to about 1800 milliseconds, the majority of this period be attributable to the update cycle required by the electro-optic material. Although the duration of the unresponsive period may be reduced by removing some of the performance artefacts that increase update time, and by improving the speed of response of the electro-optic material, it is unlikely that such techniques alone will reduce the unresponsive period below about 500 milliseconds. This is still longer than is desirable for interactive applications, such example an electronic dictionary, where the user expects rapid response to user input. Accordingly, there is a need for an image updating method and controller with a reduced unresponsive period.
The aforementioned 2005/0280626 describes drive schemes which make use of the concept of asynchronous image updating (see the paper by Zhou et al., “Driving an Active Matrix Electrophoretic Display”, Proceedings of the SID 2004) to reduce substantially the duration of the unresponsive period. The method described in this paper uses structures already developed for gray scale image displays to reduce the unresponsive period by up to 65 percent, as compared with prior art methods and controllers, with only modest increases in the complexity and memory requirements of the controller.
More specifically, the aforementioned 2005/0280626 describes two methods for updating an electro-optic display having a plurality of pixels, each of which is capable of achieving at least two different gray levels. The first method comprises:                (a) providing a final data buffer arranged to receive data defining a desired final state of each pixel of the display;        (b) providing an initial data buffer arranged to store data defining an initial state of each pixel of the display;        (c) providing a target data buffer arranged to store data defining a target state of each pixel of the display;        (d) determining when the data in the initial and final data buffers differ, and when such a difference is found updating the values in the target data buffer by (i) when the initial and final data buffers contain the same value for a specific pixel, setting the target data buffer to this value; (ii) when the initial data buffer contains a larger value for a specific pixel than the final data buffer, setting the target data buffer to the value of the initial data buffer plus an increment; and (iii) when the initial data buffer contains a smaller value for a specific pixel than the final data buffer, setting the target data buffer to the value of the initial data buffer minus said increment;        (e) updating the image on the display using the data in the initial data buffer and the target data buffer as the initial and final states of each pixel respectively;        (f) after step (e), copying the data from the target data buffer into the initial data buffer; and        (g) repeating steps (d) to (f) until the initial and final data buffers contain the same data.        
The second comprises:                (a) providing a final data buffer arranged to receive data defining a desired final state of each pixel of the display;        (b) providing an initial data buffer arranged to store data defining an initial state of each pixel of the display;        (c) providing a target data buffer arranged to store data defining a target state of each pixel of the display;        (d) providing a polarity bit array arranged to store a polarity bit for each pixel of the display;        (e) determining when the data in the initial and final data buffers differ, and when such a difference is found updating the values in the polarity bit array and target data buffer by (i) when the values for a specific pixel in the initial and final data buffers differ and the value in the initial data buffer represents an extreme optical state of the pixel, setting the polarity bit for the pixel to a value representing a transition towards the opposite extreme optical state; and (ii) when the values for a specific pixel in the initial and final data buffers differ, setting the target data buffer to the value of the initial data buffer plus or minus an increment, depending upon the relevant value in the polarity bit array;        (f) updating the image on the display using the data in the initial data buffer and the target data buffer as the initial and final states of each pixel respectively;        (g) after step (f), copying the data from the target data buffer into the initial data buffer; and        (h) repeating steps (e) to (g) until the initial and final data buffers contain the same data.        
None of the prior art described above provides a general solution to the problem of using multiple drive schemes simultaneously on a single display. In the aforementioned U.S. Pat. No. 7,119,772, only one of the two drive schemes is being applied at any one time; the monochrome or similar drive scheme is a “regional” drive scheme in the sense that it only updates the pixels which need to be changed, and thus only operates within the text box or similar selected area. If the part of the display outside the selected area needs to be changed, the display must switch back to the slower full gray scale drive scheme, so that rapid updating of the selected area is not possible which the non-selected area is being changed. Similarly, although the aforementioned 2005/0280626 provides a way of reducing the “latency” period before a new update can be started, only a single drive scheme is in use at any one time.
There is a need for a method of driving a bistable electro-optic display which permits a plurality of drive schemes to be used simultaneously. For example, in the text box/background image example used in the aforementioned U.S. Pat. No. 7,119,772, it might often be convenient for a user to scroll through a series of images displayed in the background while making notes with a keyboard or stylus in the text box area. Also, many electro-optic displays make use of so-called “menu bar operations” in which a series of radio buttons indicate which item on a menu is selected, and in such operations it is important that the radio button area be rapidly updated to that the user does not accidentally choose the wrong selection. It is also highly desirable that the method of driving a bistable electro-optic display permit the simultaneous use of multiple drive schemes having different update periods (for example, a monochrome drive scheme typically has a shorter update period than a gray scale drive scheme), and that each of the multiple drive schemes be permitted to start rewriting of its portion of the display independently of the other drive schemes; the usefulness of a rapid monochrome drive scheme is updating a menu bar is greatly diminished if a new update with the rapid monochrome drive scheme can only commence after completion of a much slower gray scale drive scheme update of a background area. The present invention provides a data structure, method of driving a bistable electro-optic display and an electro-optic display which meets these requirements.